Genetic engineering

Photo by: Gernot Krautberger

Genetic engineering is any process by which genetic material (the building
blocks of heredity) is changed in such a way as to make possible the
production of new substances or new functions. As an example, biologists
have now learned how to transplant the gene that produces light in a
firefly into tobacco plants. The function of that gene—the
production of light—has been added to the normal list of functions
of the tobacco plants.

The chemical structure of genes

Genetic engineering became possible only when scientists had discovered
exactly what is a gene. Prior to the 1950s, the term gene was used to
stand for a unit by which some genetic characteristic was transmitted from
one generation to the next. Biologists talked about a "gene"
for hair color, although they really had no idea as to what that gene was
or what it looked like.

That situation changed dramatically in 1953. The English chemist Francis
Crick (1916– ) and the American biologist James Watson
(1928– ) determined a chemical explanation for a gene. Crick and
Watson discovered the chemical structure for large, complex molecules that
occur in the nuclei of all living cells, known as deoxyribonucleic acid
(DNA).

DNA molecules, Crick and Watson announced, are very long chains or units
made of a combination of a simple sugar and a phosphate group.

Words to Know

Amino acid:
An organic compound from which proteins are made.

DNA (deoxyribonucleic acid):
A large, complex chemical compound that makes up the core of a
chromosome and whose segments consist of genes.

Gene:
A segment of a DNA molecule that acts as a kind of code for the
production of some specific protein. Genes carry instructions for the
formation, functioning, and transmission of specific traits from one
generation to another.

Gene splicing:
The process by which genes are cut apart and put back together to
provide them with some new function.

Genetic code:
A set of nitrogen base combinations that act as a code for the
production of certain amino acids.

Host cell:
The cell into which a new gene is transplanted in genetic engineering.

Human gene therapy (HGT):
The application of genetic engineering technology for the cure of
genetic disorders.

Nitrogen base:
An organic compound consisting of carbon, hydrogen, oxygen, and
nitrogen arranged in a ring that plays an essential role in the
structure of DNA molecules.

Plasmid:
A circular form of DNA often used as a vector in genetic engineering.

Protein:
Large molecules that are essential to the structure and functioning of
all living cells.

Recombinant DNA research (rDNA research):
Genetic engineering; a technique for adding new instructions to the DNA
of a host cell by combining genes from two different sources.

Vector:
An organism or chemical used to transport a gene into a new host cell.

Attached at regular positions along this chain are nitrogen bases.
Nitrogen bases are chemical compounds in which carbon, hydrogen, oxygen,
and nitrogen atoms are arranged in rings. Four nitrogen bases occur in
DNA: adenine (A), cytosine (C), guanine (G), and thymine (T).

The way in which nitrogen bases are arranged along a DNA molecule
represents a kind of genetic code for the cell in which the molecule
occurs. For example, the sequence of nitrogen bases T-T-C tells a cell
that it should make the amino acid known as lysine. The sequence C-C-G, on
the other hand, instructs the cell to make the amino acid glycine.

A very long chain (tens of thousands of atoms long) of nitrogen bases
tells a cell, therefore, what amino acids to make and in what sequence to
arrange those amino acids. A very long chain of amino acids arranged in a
particular sequence, however, is what we know of as a protein. The
specific sequence of nitrogen bases, then, tells a cell what kind of
protein it should be making.

Furthermore, the instructions stored in a DNA molecule can easily be
passed on from generation to generation. When a cell divides (reproduces),
the DNA within it also divides. Each DNA molecule separates into two
identical parts. Each of the two parts then makes a copy of itself. Where
once only one DNA molecule existed, now two identical copies of the
molecule exist. That process is repeated over and over again, every time a
cell divides.

This discovery gave a chemical meaning to the term gene. According to our
current understanding, a specific arrangement of nitrogen bases forms a
code, or set of instructions, for a cell to make a specific protein. The
protein might be the protein needed to make red hair, blue eyes, or
wrinkled skin (to simplify the possibilities). The sequence of bases,
then, holds the code for some genetic trait.

Gene splicing

The Crick-Watson discovery opened up unlimited possibilities for
biologists. If genes are chemical compounds, then they can be manipulated
just as any other kind of chemical compound can be manipulated. Since DNA
molecules are very large and complex, the actual task of manipulation may
be difficult. However, the principles involved in working with DNA
molecule genes is no different than the research principles with which all
chemists are familiar.

For example, chemists know how to cut molecules apart and put them back
together again. When these procedures are used with DNA molecules, the
process is known as gene splicing. Gene splicing is a process that takes
place naturally all the time in cells. In the process of division or
repair, cells routinely have to take genes apart, rearrange their
components, and put them back together again.

Scientists have discovered that cells contain certain kinds of enzymes
that take DNA molecules apart and put them back together again.
Endonucleases, for example, are enzymes that cut a DNA molecule at some
given location. Exonucleases are enzymes that remove one nitrogen base
unit at a time. Ligases are enzymes that join two DNA segments together.

It should be obvious that enzymes such as these can be used by scientists
as submicroscopic scissors and glue with which one or more DNA molecules
can be cut apart, rearranged, and the put back together again.

Genetic engineering procedures

Genetic engineering requires three elements: the gene to be transferred, a
host cell into which the gene is inserted, and a vector to bring about the
transfer. Suppose, for example, that one wishes to insert the gene for
making insulin into a bacterial cell. Insulin is a naturally occurring
protein made by cells in the pancreas in humans and other mammals. It
controls the breakdown of complex carbohydrates in the blood to glucose.
People whose bodies have lost the ability to make insulin become diabetic.

The first step in the genetic engineering procedure is to obtain a copy of
the insulin gene. This copy can be obtained from a natural source

DNA being injected into a mouse embryo.
(Reproduced by permission of

Phototake

.)

(from the DNA in a pancreas, for example), or it can be manufactured in a
laboratory.

The second step in the process is to insert the insulin gene into the
vector. The term vector means any organism that will carry the gene from
one place to another. The most common vector used in genetic engineering
is a circular form of DNA known as a plasmid. Endonucleases are used to
cut the plasmid molecule open at almost any point chosen by the scientist.
Once the plasmid has been cut open, it is mixed with the insulin gene and
a ligase enzyme. The goal is to make sure that the insulin gene attaches
itself to the plasmid before the plasmid is reclosed.

The hybrid plasmid now contains the gene whose product (insulin) is
desired. It can be inserted into the host cell, where it begins to
function just like all the other genes that make up the cell. In this
case, however, in addition to normal bacterial functions, the host cell
also is producing insulin, as directed by the inserted gene.

Notice that the process described here involves nothing more in concept
than taking DNA molecules apart and recombining them in a different
arrangement. For that reason, the process also is referred to as
recombinant DNA (rDNA) research.

Applications of genetic engineering

The possible applications of genetic engineering are virtually limitless.
For example, rDNA methods now enable scientists to produce a number of
products that were previously available only in limited quantities. Until
the 1980s, for example, the only source of insulin available to diabetics
was from animals slaughtered for meat and other purposes. The supply was
never large enough to provide a sufficient amount of affordable insulin
for everyone who needed insulin. In 1982, however, the U.S. Food and Drug
Administration approved insulin produced by genetically altered organisms,
the first such product to become available.

Since 1982, the number of additional products produced by rDNA techniques
has greatly expanded. Among these products are human growth hormone (for
children whose growth is insufficient because of genetic problems), alpha
interferon (for the treatment of diseases), interleukin-2 (for the
treatment of cancer), factor VIII (needed by hemophiliacs for blood
clotting), erythropoietin (for the treatment of anemia), tumor necrosis
factor (for the treatment of tumors), and tissue plasminogen activator
(used to dissolve blood clots).

Genetic engineering also promises a revolution in agriculture. Recombinant
DNA techniques enable scientists to produce plants that are
resistant to herbicides and freezing temperatures, that will take longer
to ripen, and that will manufacture a resistance to pests, among other
characteristics.

Today, scientists have tested more than two dozen kinds of plants
engineered to have special properties such as these. As with other aspects
of genetic engineering, however, these advances have been controversial.
The development of herbicide-resistant plants, for example, means that
farmers are likely to use still larger quantities of herbicides. This
trend is not a particularly desirable one, according to some critics. How
sure can we be, others ask, about the risk to the environment posed by the
introduction of "unnatural," engineered plants?

The science and art of animal breeding also are likely to be
revolutionized by genetic engineering. For example, scientists have
discovered that a gene in domestic cows is responsible for the production
of milk. Genetic engineering makes it possible to extract that gene from
cows who produce large volumes of milk or to manufacture that gene in the
laboratory. The gene can then be inserted into other cows whose milk
production may increase by dramatic amounts because of the presence of the
new gene.

Human gene therapy

One of the most exciting potential applications of genetic engineering
involves the treatment of human genetic disorders. Medical scientists know
of about 3,000 disorders that arise because of errors in an
individual's DNA. Conditions such as sickle-cell anemia, Tay-Sachs
disease, Duchenne muscular dystrophy, Huntington's chorea, cystic
fibrosis, and Lesch-Nyhan syndrome result from the loss, mistaken
insertion, or change of a single nitrogen base in a DNA molecule. Genetic
engineering enables scientists to provide individuals lacking a particular
gene with correct copies of that gene. If and when the correct gene begins
functioning, the genetic disorder may be cured. This procedure is known as
human gene therapy (HGT).

The first approved trials of HGT with human patients began in the 1980s.
One of the most promising sets of experiments involved a condition known
as severe combined immune deficiency (SCID). Individuals with SCID have no
immune systems. Exposure to microorganisms that would be harmless to the
vast majority of people will result in diseases that can cause death.
Untreated infants born with SCID who are not kept in a sterile bubble
become ill within months and die before their first birthday.

In 1990, a research team at the National Institutes of Health (NIH)
attempted HGT on a four-year-old SCID patient. The patient received about
one billion cells containing a genetically engineered copy of the
gene that his body lacked. Another instance of HGT was a procedure,
approved in 1993 by NIH, to introduce normal genes into the airways of
cystic fibrosis patients. By the end of the 1990s, according to the NIH,
more than 390 gene therapy studies had been initiated. These studies
involved more than 4,000 people and more than a dozen medical conditions.

In 2000, doctors in France claimed they had used HGT to treat three babies
who suffered from SCID. Just ten months after being treated, the babies
exhibited normal immune systems. This marked the first time that HGT had
unequivocally succeeded.

Controversy remains.
Human gene therapy is the source of great controversy among scientists
and nonscientists alike. Few individuals maintain that the HGT should not
be used. If we could wipe out sickle cell anemia, most agree, we should
certainly make the effort. But HGT raises other concerns. If scientists
can cure genetic disorders, they can also design individuals in accordance
with the cultural and intellectual fashions of the day. Will humans know
when to say "enough" to the changes that can be made with
HGT?

Genetically engineered sheep with the human gene responsible for the
production of alpha-1-antitrypsin (AAT) transferred into their DNA.
When the AAT is eventually extracted from their milk, it will be used
as therapy for humans deficient in AAT. The deficiency causes
emphysema (a breathing disorder) in approximately 100,000 people in
the western world.
(Reproduced by permission of

Photo Researchers, Inc.

)

Despite recent successes, most results in HGT since the first experiment
was conducted in 1990 have been largely disappointing. And in 1999,
research into HGT was dealt a blow when an eighteen-year-old from Tucson,
Arizona, died in an experiment at the University of Pennsylvania. The
young man, who suffered from a metabolic disorder, had volunteered for an
experiment to test gene therapy for babies with a fatal form of that
disease. Citing the spirit of this young man, researchers remain
optimistic, vowing to continue work into the possible lifesaving
opportunities offered by HGT.

The commercialization of genetic engineering

The commercial potential of genetically engineered products was not lost
on entrepreneurs in the 1970s. A few individuals believed that the impact
of rDNA on American technology would be comparable to that of computers in
the 1950s. In many cases, the first genetic engineering firms were founded
by scientists involved in fundamental research. The American biologist
Herbert Boyer, for example, teamed up with the venture capitalist Robert
Swanson in 1976 to form Genentech (Genetic Engineering Technology). Other
early firms like Cetus, Biogen, and Genex were formed similarly through
the collaboration of scientists and businesspeople.

The structure of genetic engineering (biotechnology) firms has, in fact,
long been a source of controversy. Many observers have questioned the
right of a scientist to make a personal profit by running companies that
benefit from research that had been carried out at publicly funded
universities. The early 1990s saw the creation of formalized working
relations between universities, individual researchers, and the
corporations founded by these individuals. Despite these arrangements,
however, many ethical issues remain unresolved.

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